Le contexte : Un article sur un recueil de nouvelles de TC Boyle : "Written in a sombre tone at odds with the blackly satirical style of the other 12 stories, it serves only to underscore how compelling Boyle can be when he lets the beast out."

I have never heard this expression before, but it probably refers to 'the beast within' idea which has been picked up in recent horror and then popular films and literature. It originally meant the resident evil within a person. Its negative connotations have lost some of their strength due to the current trend of fascination for things previously considered evil, such as vampires, for example.